1. Field of the Invention
The present invention relates to a tire vulcanizing mold for vulcanizing and molding a tire and a process for manufacturing the same.
2. Description of the Prior Art
To form a tire, a vulcanizing mold is used in which pressure is applied to the inside of the molded green tire to pressure contact the outer surface of the above green tire to the inner wall of the heated mold in order to vulcanize the crude rubber by heat and pressure.
The above vulcanizing mold is a full type mold which is cast for a whole tire integrally or a split type mold 50 consisting of a plurality of sector molds 54, each comprising a crown mold 51 having a sectional form corresponding to a tire crown portion and side molds 52 mounted to a holder 53, all of which are interconnected in a loop as shown in FIGS. 11(a) and 11(b). A piece type tire mold 60 consisting of a plurality of sector molds 65, each comprising upper and lower molds 61 and 62 and pieces 64 fixed to a holder 63, arranged in a loop in the circumferential direction of a tire as shown in FIGS. 12(a) and 12(b) is employed in large quantities. The above piece 64 is generally cast by a die cast method in which a molten metal is poured into a metal mold at a high temperature and a high pressure to be cast for each pitch of a tread pattern and these pieces are combined together according to a pitch variation and mounted to the above holder 63.
In general, to discharge air in the inside of a mold or a gas generated by vulcanization to the outside of the mold, an air bleeder through hole called “vent hole”, or a hole or pore having a very small diameter such as an air vent or exhaust passage for gassing is formed in the above crown mold 51 and the piece 64. However, a rubber material flows into the above vent hole at the time of vulcanizing and molding a tire so that a rubber projection called “spew” is formed on the surface of a product tire. Therefore, the work of removing the spew is required after molding.
In the method in which the air vent or exhaust passage is formed, as the above air vent or the exhaust passage is small in size and a surface projection having a complex shape is existent therearound, it is difficult to form a hole or pore having a desired diameter. For example, in the case of milling, as the width of a slit to be formed is very small, the cutter strength and the processing depth are limited and the processing time becomes very long. In the case of discharge processing, the manufacture of an electrode is necessary, and the processing depth is limited due to limitations such as the removal of cutting powders and the curvature of the electrode. Laser beam processing making use of a laser such as CO2 laser or YAG laser is conceivable but the processing depth is limited due to the focusing distance of a laser beam when the width of a groove is 0.1 mm or less.
Since the size of the above air vent or the exhaust passage is limited by a tool in use, a very small hole or pore cannot be formed with high accuracy. As a result, a rubber material which becomes a fluid by vulcanizing heat at the time of molding a tire enters the above hole or pore and a large number of spews are formed on the surface of the vulcanized tire, thereby impairing the appearance of the tire, or clogging occurs due to the cuttings of the spews in the vulcanizing mold.
To cope with these, there is proposed a process for directly manufacturing a tire vulcanizing mold using a 3-D image of a tire to be manufactured without using a tool (for example, refer to JP-A 10-244540) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). In this method, a tire vulcanizing mold is manufactured by a powder sintering method in which sinterable powders of a metal material or ceramic-based material are heated and sintered by heating means to form a laminate of layers as at least part or all of the mold. The apparatus used in this method is, for example, a mold laser sintering apparatus 70 marketed by EOS Co., Ltd. or 3D Systems (of the U.S.) as shown in FIG. 13. In this apparatus 70, powder particles 72 having an average particle size of 30 to 100 μm are stored in a holding chamber 71, a lifting plate 73 in the above holding chamber 71 is moved up by a predetermined amount to take out a powder layer 72L having a thickness of 0.2 to 0.5 mm, and this layer 72L is transferred into a collection chamber 75 installed next to the holding chamber 71 by a distribution doctor blade 74 and heated and sintered by a laser beam 76z from a laser apparatus 76 as local heating means so as to form a laminated sintered body.
The direction of a mirror 78 for controlling the optical path of the laser beam 76z is controlled by an electronic controller 77 based on the prestored CAD drawing of a tire, the above powder layer 72L is scanned with the above laser beam 76z to be sintered within the space whose boundary is set to a predetermined contour so as to form each layer of the laminated sintered body having the above predetermined contour. By repeating this step, a tire vulcanizing mold composed of a laminated sintered body having a fine projection or an extremely small-diameter hole or pore such as an air vent or exhaust passage which has been difficult to machine can be manufactured.
High strength enough to stand the above vulcanizing conditions is required for the tire vulcanizing mold because a plurality of sector molds are tightly secured at a high pressure at the time of molding.
Although a member having a complex shape can be obtained in a tire vulcanizing mold manufactured by the above powder sintering method, as the sintered portion is porous, it has lower density than a conventional cast type mold and cannot obtain sufficiently high strength required for a tire vulcanizing mold.
Meanwhile, when the above tire vulcanizing mold is manufactured by casting, mechanical processing or a combination of casting and mechanical processing, the number of processing steps is large and the strength of a tire vulcanizing mold 80 is uniform as shown in FIG. 14(a) as the same material is used, thereby making it difficult to provide high strength only to a desired portion. Therefore, to secure required strength, the strength of the whole tire mold must be increased with result of a lot of waste.
As shown in FIG. 14(b), a hybrid structure that a high strength material such as iron is used in the mating portions 81 of the tire vulcanizing mold 80 and aluminum is used in other portions including a tire crown portion 82 in consideration of weight and heat conductivity is employed. However, this further increases the number of processing steps and boosts costs.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a process for manufacturing a tire vulcanizing mold having sufficiently high strength and durability efficiently, which is capable of forming a portion having a complex shape of the inner wall of a mold and an air vent or exhaust passage with high accuracy.